1. Field of the Invention
The present invention relates to a Doppler frequency estimating device that estimates a Doppler frequency in a communication system, a receiving device, a recording medium, and a Doppler frequency estimating method.
2. Description of the Related Art
In recent years, orthogonal frequency division multiplexing (OFDM) having a high frequency utilization efficiency is adopted to realize a high-speed wireless communication system (Document 1).
An OFDM system attracts attention as a technology that realizes high-speed wireless communication, and the demand for the OFDM system is high. The OFDM system is utilized for a wireless LAN based on IEEE 802.11a/g/n or the like, a wireless MAN such as WiMAX, or terrestrial digital broadcasting.
On the other hand, with the rapid spread of wireless communication systems, a frequency utilization efficiency may become a problem in some cases. Thus, a radio equipment is provided with a function of recognizing a peripheral radio wave environment, and realizing cognitive radio that selects an optimum communication scheme in accordance with such an environment is expected (Documents 2-4). At the present day, in the cognitive radio, the study for detecting a vacant frequency band and dynamically distributing a frequency is mainly examined (Document 5). The technology that changes not only a frequency band but also various parameters in a communication system, e.g., a modulation scheme or a transmission rate in accordance with a channel environment to thereby improve a transmission efficiency is also suggested (Document 6).
As indexes for a channel state, there are, e.g., an SNR (Signal Noise Ratio), a multipath profile, a delay spread, a maximum delay time of a delay wave, a Doppler frequency, and others. In particular, in mobile wireless communication, fading due to Doppler becomes a problem in a channel environment. A channel becomes a non-stationary because of an influence of Doppler, and a channel coherence time is decreased with an increase in a maximum Doppler frequency. As a result, a degradation in channel estimation accuracy in a receiver, i.e., ICI (Inter-channel Interference) occurs, which causes degradation in a communication quality in some cases. Therefore, a study for adaptively selecting an FFT (Fast Fourier Transform) point number by using an estimated maximum Doppler frequency is conducted (Document 7).
However, to adopt such a technique, a sufficient estimation accuracy for a maximum Doppler frequency is required.
It has been conventionally known that a relationship between an autocorrelation value of Rayleigh fading and a maximum Doppler frequency can be approximated by a Bessel function, and a technique for estimating the maximum Doppler frequency by using such characteristics is suggested (Document 7).
Further, Document 8 discloses a correlation estimating technique in a CDMA scheme.
In Document 7 that discloses a conventional technique, the Bessel function indicative of the relationship between the autocorrelation value of a Rayleigh wave and the maximum Doppler frequency is utilized to estimate the maximum Doppler frequency. In this technique, to estimate the maximum Doppler frequency for a channel, an autocorrelation value based on time averaging of the Rayleigh wave must be estimated. According to this conventional technique, the autocorrelation value alone of a path having an SNR that becomes maximum when the channel is a multipath is used, ergodic properties are presupposed for the estimation of the autocorrelation, and a time average value based on a single path is utilized to estimate the maximum Doppler frequency. According to this conventional technique, an estimation accuracy for the maximum Doppler frequency may be degraded.
Furthermore, the convergence rate of the estimation is decreased.
Moreover, in Document 8, a method that utilizes a multipath is suggested as the CDMA maximum Doppler frequency estimating method, but an accuracy for separating the multipath is low, obtaining a sufficient estimation accuracy for the maximum Doppler frequency is difficult, and a path selection technique is not examined. Therefore, it is difficult to apply Document 8 to, e.g., a communication system adopting an OFDM scheme or the like based on a multipath communication environment.
[Document 1] Makoto Itami, “Comprehensible OFDM technology”, Ohmsha Ltd., Tokyo, 2005
[Document 2] Joseph Mitola III and Gerald Q. Maguire, Jr., “Cognitive Radio: Making Software Radios More Personal,” IEEE Personal Communications, Vol. 6, No. 4, pp. 13-18, August 1999
[Document 3] Simon Haykin, “Cognitive Radio: Brain-Empowered Wireless Communications,” IEEE Journal on Selected Areas in Communications, Vol. 23, No. 2, February 2005
[Document 4] Joseph Mitola III, Cognitive Radio Architecture: The Engineering Foundations of Radio XML, Wiley-Interscience, New Jersey, 2006
[Document 5] Junji Yamamoto, Ryouji Yamaoka, Seishi Hanaoka, Satoshi Yoshizawa, Tetsuhiko Hirata, “Proposal of Terminal Assignment Method for Cognitive Radio”, Shingaku Giho, SR2006-41, November 2006
[Document 6] Bruce A. Fette, Cognitive Radio Technology, Newnes, 2006
[Document 7] Takayuki Sugawara, Shingo Miyazawa, Kiichi Miyanaga, “A Consideration Concerning Environment-adaptive OFDM system”, Shingaku Giho, SR2006-57, November 2006
[Document 8] JP-A 11-234190 (KOKAI)